One known Coherent Beam Combining system (CBC) is based on a sparse multi-aperture array of fiber optic collimators. As a metric for external active control of phase-locking a technique called the power in the bucket (PIB) is used. The PIB technique uses the intensity of the photons returned from a target for feedback control of phase shifters that control the phase of the laser beam sources. FIG. 1 illustrates such a system.
The travel time of the return photons from the target is variable due to reliance of target reflection and atmospheric conditions, which may cause time delays that prevent optimization of the phase locking performance.
Preliminary phase locking of the source beams can solve these drawbacks using for instance beam splitters in the train of output laser beams, as shown in FIG. 2. Here, the CBC system with external phase-locking uses portions of beamlets split from the power train by means of beam splitters placed in a near-field of the laser beam power train. The overlapping of these beamlet portions near the focal plane of a focusing lens yields constructive interference spots, at least one of which is selected by use of a pinhole. A photo-sensor placed behind the pinhole is used to indicate an intensity for the selected spot, which intensity is used as a metric for active feedback control of the phase of the source beams. However, use of beam splitters which are external to the fiber collimators are disadvantageous in that they can cause wave front power aberrations in the laser beams, they are bulky, and delicate elements make the system heavy and non-reliable, especially for mobile applications.
FIGS. 3A and 3B show side and end schematic views of an arrangement that uses the internal photons of constituent laser beams, that is, before the photons reach the output collimating lenses. These photons are intercepted in periphery areas of the divergent (Gaussian) beams, which are parasitic beam-tails that remain inside of the beam array. More specifically, for internal phase-locking of wave fronts of an array of fiber optic collimators, the periphery areas of Gaussian beams (i.e., beam tails) are used, which beam tails, in one embodiment, are clipped before reaching the output collimating lenses. FIG. 3A illustrates the internal phase-locking of neighbor fiber optic collimators 101 and 102 using beam tails 110-2 and 120-1 of Gaussian beams 110 and 120. Mirrors or diffractive optic elements (DOE) 600-1 and 600-2 intercept/clip the beam tails 110-2 and 120-1 and re-direct them to the back of the array where they are focused on a plane near a pinhole photodetector. The pinhole selects the constructive interference spot of these beams after their overlap near the focal plane. An intensity signal from the photodiode placed behind of the pinhole provides a metric for internal feedback phase-locking of neighbor beamlets. A simplest example includes two channels as shown in FIG. 3A. In the case of hexagon packing of sub-apertures, instead of two, three mirrors 600-1, 600-2, 600-3 are used to intercept the beam tails of three adjacent sub-apertures 210, 220, 230.
Drawbacks of the internal phase-locking:                Diffractive optic elements (DOE) or precision assembly of three parabolic sub-mirrors or mini-holograms (in case of hexagonal beamlet packaging) intercepting the Gaussian beam tails, are complicated and expensive optical devices.        DOEs or assemblies of sub-mirrors are inside of array, causing the problems of precision alignment and possible thermal aberration in case of high beams power.        The size of interference spot behind of array is very small (typically 5-15 microns), causing the problem of alignment and stability of the pinhole with small diameter.        